The present invention relates to a vacuum generator, which is used for, for example, a conveying device capable of holding a work piece by air suction, more precisely relates to a vacuum generator capable of reducing amount of compressed air and efficiently using compressed air.
A vacuum generator is assembled in a conveying device which holds a work piece by air suction. In the conventional vacuum generator, a vacuum state or a negative pressure state is generated in a vacuum port by using compressed air. The vacuum state is generated and disappeared by a switching valve, which controls the supply of compressed air. A work piece is sucked to the vacuum port when the vacuum state is generated in the vacuum port.
A sectional view of a conventional vacuum generator is shown in FIG. 6. The vacuum generator comprises: an air-supply port 10 to which compressed air is supplied; an air-discharge port 40 from which compressed air is discharged; and a vacuum port 50 in which a vacuum state or a negative pressure state is generated so as to hold a work piece. A main valve 60 is moved in the axial direction by a pilot valve 70. Communication between an air-supply path 12 and a first communication path 14 is controlled on the basis of positions of the main valve 60. While the air-supply path 12 and the first communication path 14 are communicated, the vacuum state is generated and the work piece can be held by air suction; while the air-supply path 12 and the first communication path 14 are not communicated, the vacuum state is disappeared and the work piece can be released.
A nozzle 18 is provided in a cylinder 16, and a diffuser nozzle 20 is provided on the front side of the nozzle 18. Compressed air introduced via the first communication path 14 is jetted from the nozzle 18, so that the vacuum state is generated in the vacuum port 50. A cylinder 52 is communicated to the cylinder 16 via a communication path 45. By jetting the compressed air from the nozzle 18 toward the diffuser nozzle 20, air is sucked through the cylinder 52 and the communication path 45, so that the work piece is sucked to the vacuum port 50.
To efficiently convey work pieces, the vacuum generator must hold and release the work piece in a short time. Holding and releasing work pieces are influenced by response and vacuum characteristics of the vacuum port. To quickly suck and hold the work piece, amount of sucking air must be large. However, a large amount of compressed air must be required so as to suck a large amount of air.
Conventionally, the vacuum generator is selected on the basis of following conditions: total capacity of a vacuum generating section including tubes, amount of compressed air to be consumed, capacity of a compressor, leakage from a connecting part between the work piece and an actuator, etc. However, the conditions are considered for sucking the work piece; amount of compressed air for holding the work piece is not considered. As described above, the work piece can be quickly and securely sucked to the vacuum port by sucking a large amount of air. However, after the work piece is once held, the work piece can be fully held by sucking a small amount of air, which supplements leakage of air in a vacuum circuit. Therefore, after the work piece is once held, amount of consuming compressed air can be reduced by reducing amount of air sucked. In the case of a vacuum generator whose nozzle has a large diameter, the amount of sucking air is large. And, in the case of a conveying device which takes a long time to convey the work piece, it is advantageous for energy reduction to reduce the amount of consuming compressed air.
Another conventional vacuum generator capable of sucking a large amount of air from a vacuum port is known. In the vacuum generator, a first ejector unit whose nozzle has a small diameter and a second ejector unit whose nozzle has a large diameter are arranged in series. The vacuum generator is capable of sucking a large amount of air, but amount of consuming compressed air is not reduced.
Further, a vacuum generator capable of reducing amount of consuming compressed air is known. In the vacuum generator, a first ejector unit, which is capable of generating a low degree vacuum state, and a second ejector unit, which is capable of generating a high degree vacuum state, are arranged in parallel. The ejector units are selectively actuated (see Japanese Patent Gazette No. 61-55399). However, by employing two ejector units, number of parts must be increased, and the vacuum generator must be large-sized.